The present invention relates to arrayed waveguide gratings, and more particularly to systems and methods for controlling both the dispersion and passband characteristics of arrayed waveguide gratings.
Dispersion of a material relates to the speed that energy at different wavelengths travels through a material. Controlling the dispersion of arrayed waveguide gratings (AWGs) will be essential in 40 Gigabit per second (Gbps) transmission systems and in spectrally efficient 10 Gbps systems (50 GHz spacing). Having the ability to design the dispersion characteristics in an AWG offers significant advantages.
AWGs typically include an input waveguide, a slab free-propagation region, an arrayed waveguide grating, a slab-focusing region and output waveguides. Conventional AWG designs have obtained a flattened AWG intensity passband by applying a multi-modal structure (MMI, parabolic horn, etc . . . ) to the standard AWG design at the entrance of one slab and a standard taper on the other slab. Because the passband is the result of the convolution between the two modes, the spectrum gets flattened. However, this design method leads to high dispersion and dispersion slope in the passband due to the non-flat phase of the multi-modal structure.
A second approach modifies the arrayed waveguide gratings to generate a sinc-like distribution that leads to a dispersion-free flattened passband. This approach uses a standard input/output in the slabs but modifies the amplitude distribution by adding some loss on some waveguides (amplitude) and changing the length of some waveguides (phase).
In U.S. Pat. No. 6,195,482 to Dragone, which is hereby incorporated by reference, the design of an arrayed waveguide grating is optimized by adjusting arm lengths and utilizing multi-mode input and output couplers to produce a specified passband behavior. Control of dispersion of AWGs is not discussed.
A method according to the invention for designing an arrayed waveguide grating that includes input and output couplers, input and output slabs, and a plurality of arms connecting the input and output slabs includes the step of determining a desired amplitude response for the arrayed waveguide grating. A desired dispersion response for the arrayed waveguide grating is determined. Input and output couplers are designed to produce the desired amplitude response. The lengths of the arms of the arrayed waveguide grating are perturbed to produce a flat or linear dispersion.
In another aspect of the invention, an arrayed waveguide grating includes input and output couplers, input and output slabs, and a plurality of arms connecting the input and output slabs. The input and output couplers produce a desired amplitude response for the arrayed waveguide grating. A length of the arms is adjusted to produce a flat or linear dispersion.
In yet another aspect of the invention, an arrayed waveguide grating includes a parabolic input coupler, a parabolic output coupler and an input slab connected to the parabolic input coupler. An output slab is connected to the parabolic output coupler. A plurality of arms connect the input and output slabs. The parabolic input and output couplers produce a flat passband response. Individual lengths of the arms are adjusted to produce a flat dispersion response.
In still another aspect of the invention, a method for changing the sign of a linear dispersion response of an arrayed waveguide grating includes the steps of connecting an input coupler to an input slab. An output coupler is connected to an output slab. The input and output slabs are connected using a plurality of waveguide arms. Individual lengths of the waveguide arms are adjusted to reverse the sign of a group delay, dispersion and dispersion slope of the arrayed waveguide grating.
In yet another aspect of the invention, an optical system includes a multiplexer that multiplexes a plurality of optical signals that are input to the multiplexer and outputs a multiplexed optical signal. The multiplexer has a first group delay, a first dispersion and a first dispersion slope. The first group delay, the first dispersion and the first dispersion slope have one of a positive and negative polarity. A demultiplexer demultiplexes the multiplexed optical signal that is input to the demultiplexer and outputs the plurality of optical signals. The demultiplexer has a second group delay, a second dispersion and a second dispersion slope. The second group delay, the second dispersion and the second dispersion slope have the other of a positive and negative polarity. The first and second group delay, dispersion and dispersion slope substantially cancel.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.